1. Field of the Invention
The present invention relates generally to reconstitutable fuel assemblies for nuclear reactors and, more particularly, is concerned with a reconstitutable nuclear fuel assembly having a top-nozzle-to-guide-thimble attachment system employing reusable locking tubes with preformed dimples.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission. The reactor also has control rods which can be inserted into the guide thimbles to control the fission reaction. The fission reaction releases a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
During operation in the nuclear reactor, the fuel rods may occassionally develop cracks along their lengths resulting primarily from internal stresses. These defective fuel rods must be replaced in the fuel assemblies, and this replacement must occur under water as the fuel assemblies become highly radioactive during their operation in the reactor. To gain access to a defective fuel rod, it is necessary to remove the top and/or bottom nozzle of the fuel assembly. Reconstitutable fuel assemblies exist which are designed with removable nozzles. Typical removable top (bottom) nozzles have been attached to the top (bottom) of the guide thimbles using a threaded arrangement. Typical removable top nozzles also have been attached to the top of the guide thimbles using a bulge/groove arrangement, including the use of locking tubes, such as disclosed in U.S. Pat. No. 4,631,168, hereby incorporated by reference.
Commonly owned U.S. patent application Ser. No. 719,108 entitled "Reusable Locking Tube in a Reconstitutable Fuel Assembly" by John M. Shallenberger et al., filed Apr. 2, 1985 (W.E. 52,507), is hereby incorporated by reference. The invention disclosed therein is a reconstitutable nuclear fuel assembly having reusable locking tubes with preformed dimples, and it has been in use and on sale in the U.S. for more than one year. Some locking tubes in those fuel assemblies were found to be seated too low. A study revealed that this problem had two possible causes. A properly seated locking tube could become improperly seated too low due to the force exerted on it when the fuel assembly was lifted, such as at the reactor site at the time of loading the fuel assembly into the reactor core. Also, a locking tube easily could be seated too low at the time of installation. A low-seated locking tube would require special (longer-handled) tooling for locking tube removal in the event the fuel assembly required reconstitution. This would complicate the underwater reconstitution operation.
What is needed in a removable locking tube design which would insure proper seating of the locking tube at the time of installation and after fuel assembly lifting, and which would avoid time-consuming measurement checks at the factory and at the reactor.